Ozone exposure and stomatal sluggishness in different plant physiognomic classes

Gas exchange responses to static and variable light were tested in three species: snap bean ( Phaseolus vulgaris, two cultivars), California black oak ( Quercus kelloggii), and blue oak ( Q. douglasii). The effects of 1-month (snap beans) and 2-month (oaks) O 3 (ozone) exposure (70 ppb over 8 h per day in open-top chambers) were investigated. A delay in stomatal responses (i.e., ‘sluggish’ responses) to variable light was found to be both an effect of O 3 exposure and a reason for increased O 3 sensitivity in snap bean cultivars, as it implied higher O 3 uptake during times of disequilibrium. Sluggishness increased the time to open (thus limiting CO 2 uptake) and close stomata (thus increasing transpirational water loss) after abrupt changes in light level. Similar responses were shown by snap beans and oaks, suggesting that O 3-induced stomatal sluggishness is a common trait among different plant physiognomic classes. Sluggish stomatal responses are suggested to be both an effect of O 3 exposure and a reason of increased O 3 sensitivity in plants.